Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of roller cone (xe2x80x9crockxe2x80x9d) bits used to drill wellbores through earth formations. More specifically, the invention is related to structures for the roller cone for single cone rock bits.
2. Background Art
Roller cone bits are one type of drill bit used to drill wellbores through earth formations. Roller cone bits include a bit body adapted to be coupled to a drilling tool assembly or xe2x80x9cdrill stringxe2x80x9d which rotates the bit as it is pressed axially into the formations being drilled. The bit body includes one or more legs, each having thereon a bearing journal. The most commonly used types of roller cone drill bits include three such legs and bearing journals. A roller cone is rotatably mounted to the bearing journal. During drilling, the roller cones rotate about the respective journals while the bit is rotated. The roller cones include a number of cutting elements, which may be press fit inserts made from tungsten carbide and other materials, or may be milled steel teeth. The cutting elements engage the formation in a combination of crushing, gouging and scraping action which removes small segments of the formation being drilled.
One particular type of roller cone drill bit includes only one leg, bearing journal and roller cone rotatably attached thereto. The drilled hole and the longitudinal axis of this type of bit are generally concentric. This type of drill bit has generally been preferred for drilling applications when the diameter of the hole being drilled is small (less than about 4 to 6 inches [10 to 15 cm]) because the bearing structure can be larger relative to the diameter of the drilled hole when the bit only has one concentric roller cone. This is in contrast to the typical three-cone rock bit, in which each journal must be smaller relative to the drilled hole diameter. An example of a single cone rock bit is shown in U.S. Pat. No. 6,119,797 issued to Hong et al.
An important performance aspect of any drill bit is the ability of drilling fluid (xe2x80x9cmudxe2x80x9d) pumped through orifices in the bit body to clean the bit, to cool and lubricate the cutting structures on the bit and to lift cuttings out of the wellbore (xe2x80x9chole cleaningxe2x80x9d). Hole cleaning is a particular problem for single cone bits because the cone occupies substantially the entire cross-section of the drilled hole, thus not providing ample space for cuttings evacuation past the cone.
Another aspect of bit performance is the ability of the bit to avoid xe2x80x9cballingxe2x80x9d, a situation in which mud solids and formation cuttings become lodged between the individual cutting elements on the roller cone. Balling reduces the ability of the cuffing elements to penetrate formation, thereby reducing drilling performance.
Another performance aspect of a roller cone drill bit is its ability to avoid xe2x80x9ctrackingxe2x80x9d, a situation in which cutting elements traverse the same subset of the cross-section of the drilled hole, leaving other areas of the cross-section undrilled. Tracking reduces drilling performance because the hole bottom is not evenly drilled. Avoiding tracking in single cone rock bits is particularly difficult because of the very complex motion of the individual cutting elements on the roller cone as the bit drills earth formations.
Accordingly, it is desirable to have a single cone rock bit which has increased hole cleaning capacity, reduced tendency toward balling, and reduced tendency toward tracking.
One aspect of the invention is a roller cone drill bit, including a bit body adapted to be coupled to a drill string, a bearing journal depending from the bit body and a single roller cone rotatably attached to the bearing journal. The roller cone has a plurality of inserts disposed on it. The journal defines a rotation angle with respect to an axis of rotation of the bit such that the roller cone includes an intermittent contact zone and a continuous contact zone thereon. At least one of the inserts disposed in the continuous contact zone is made from a super hard material.
Another aspect of the invention is a roller cone drill bit, including a bit body adapted to be coupled to a drill string, a bearing journal depending from the bit body and a single roller cone rotatably attached to the bearing journal. The roller cone has a plurality of inserts disposed on it. The inserts have an extension length of at least 0.6 inches.
Another aspect of the invention is a roller cone drill bit, including a bit body adapted to be coupled to a drill string, a bearing journal depending from the bit body, and a single roller cone rotatably attached to the bearing journal. The roller cone has a plurality of inserts disposed thereon. The inserts are arranged in a plurality of substantially helical columns. The columns are arranged so that a plane containing the axis of the cone intersects at least one and at most two of the columns on one side of the cone surface.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.